U.S. Pat. No. 5,827,602 and U.S. Pat. No. 5,965,054 disclose non-aqueous ionic liquids for use as electrolytes that are based on salts composed of separate anion and cation moieties. The cation moieties are selected from pyridinium, pyridazinium, pyrimidinium pyrazinium, imidazolium, pyrazolium, thiazolium, oxazolium, and triazolium ions.
U.S. Pat. No. 6,090,895 discloses crosslinked polymers having imide crosslinking groups and methods of crosslinking polymers to form imide crosslinking groups. These crosslinked polymers may be useful as polymer electrolyte membranes (PEM's) in fuel cells.
U.S. Pat. Nos. 6,063,522, 4,505,997, 5,652,072, 5,072,040 and 5,514,493 disclose electrolytes comprising imide functional groups.
U.S. Pat. No. 5,463,005 discloses perfluorinated monomers and polymers comprising sulfonyl and carbonyl imide groups for use as solid polymer electrolytes.
M. Doyle, S. K. Choi and G. Proulx, J. Electrochem. Soc., 147, 34-37, discloses the use of ethyl methyl imidazolium (EMI) triflate, a low melting ionic liquid composed of separate anion and cation moieties, together with a known sulfonated fluoropolymer (Nafion) to give a fuel cell membrane material for use at elevated temperatures.
SUMMARY OF THE INVENTION
Briefly, the present invention provides a zwitterionic imide compound according to the formula: R1—SO2—N−—SO2—R2+, where R1 and R2+ are any suitable groups. Typically R1 is a highly fluorinated alkane and R2+ is a heteroatomic aromatic group having an cationic nitrogen, such as: pyridiniumyl, pyridaziniumyl, pyrimidiniumyl, pyraziniumyl, imidazoliumyl, pyrazoliumyl, thiazoliumyl, oxazoliumyl, or triazoliumyl. Typically the zwitterionic imide has a melting point of less than 100° C. Typically the zwitterionic imide compound has a solubility in water of less than 5% by weight.
In another aspect, the present invention provides a zwitterionic liquid having a melting point of less than 100° C. and typically having a solubility in water of less than 5% by weight.
In another aspect, the present invention provides an electrochemical device comprising the zwitterionic liquid or the zwitterionic imide according to the present invention, such as a battery or a fuel cell.
What has not been described in the art, and is provided by the present invention, is an ionic liquid having a melting point of less than 100° C. that is a zwitterionic compound, particularly a zwitterionic imide compound.
In this application:
“highly halogenated”, means containing halogen in an amount of 40 wt % or more, but typically 50 wt % or more, and more typically 60 wt % or more; and
“highly fluorinated”, means containing fluorine in an amount of 40 wt % or more, but typically 50 wt % or more, and more typically 60 wt % or more; and
“substituted” means, for a chemical species, substituted by conventional substituents which do not interfere with the desired product or process, e.g., substituents can be alkyl, alkoxy, aryl, phenyl, halo (F, Cl, Br, I), cyano, nitro, etc.
It is an advantage of the present invention to provide an electrolyte for use in electrochemical devices such as batteries and fuel cells at temperatures in excess of 80° C. The zwitterionic liquids can be more stable in use than conventional (non-zwitterionic) ionic liquids, since they do not form a neutral, volatile, water soluble species upon protonation of the anion.